I. Field of the Invention:
This invention relates generally to a method and apparatus for processing materials, such as semicoductor devices in a vacuum environment, and more particularly to an improved processing method and apparatus whereby low yields due to contamination are overcome.
II. Discussion of the Prior Art:
In fabricating integrated circuit chips, a wafer of semiconductor materials, such as silicon or gallium arsonide, are subjected to a sequential series of processing steps, including oxidation, dopant diffusion by ion implantation, masking and etching, metal depositions, further depositions of insulating materials, such as silicon oxide, in the formation of various junctions and interconnects including terminal pads and the like. The silicon wafer is then tested and ultimately sliced into discrete integrated circuit chips before the chips are individually encased in a suitable package. The aforementioned steps of metallization, passivation, etc., must be carried out in an extremely clean environment. Semiconductor production facilities commonly include so-called "clean rooms" in which the room air is filtered and exchanged at high flow rates and workers are gowned in relatively dust-free clothing. These ultra-clean production environments are mandated when it is considered that even the tiniest of foreign contaminants can result in a defective semiconductor device.
As indicated above, various metallization and passivation steps performed on the silicon wafers are carried out in a vacuum using vacuum deposition and vacuum sputtering techniques. Typically, during a metallization step, one or more semiconductor wafers are placed in a vacuum chamber which may be pumped down to a pressure of -5.times.10.sup.-6 torr and then the metal, typically aluminum, contained in a fixture is exposed to a high energy electron beam and caused to vaporize. The vapors are allowed to condense on the silicon substrate, and then the metal layer is later etched, in the vacuum chamber by a reactive sputtering process or otherwise, in accordance with a predetermined pattern defined by a photoresist layer which has been exposed to light through one or more masks. More than one such metallization step is usually required.
In a subsequent step, the metallized and etched wafer may be placed in still another vacuum chamber in which silicon oxide (glass) is deposited so as to create an insulating passivation layer.
A typical vacuum system to provide a local gaseous environment for use in semiconductor manufacture would typically include a process chamber having entrance and exit locks, a vacuum pump providing a means for evacuating the gaseous content of the process chamber as well as the load lock and exit lock thereto, an on/off vent valve which allows control of the vacuum displacement with a desired gas and also a on/off vacuum valve for controlling the evacuation of the process chamber by means of the vacuum pump.
The standard operation of the vacuum valve and the vent valve in a typical prior art vacuum deposition system creates a pressure "burst" when either pumping the system to a predetermined vacuum or when venting the system to atmosphere, such as occurs prior to the loading and unloading of products into and from the vacuum chamber. I have found that this pressure burst will disturb and redistribute contamination that resides within the process chamber. The contamination typically comprises minute, microscopic particles which have remained in the processing chamber following previous runs of product through that chamber. For example, after only a few cycles of operation, the vacuum metallization chamber collects a residue of metal or metallic oxides which come to rest on the fixtures contained within the vacuum chamber. Similarly, in the glass passification step, minute particles of silicon oxide can collect on the surfaces of the vacuum chamber used for that operation, later to become shaken free due to the rush or burst of gas movement when the pressure in the chamber or the locks leading thereto is suddenly changed. This redistribution of contamination can and often does result in decreased process and product yield, which, in turn, reflects a loss of profit dollars.